1. Field of the Invention
The present invention relates to air electrode bodies for solid oxide fuel cells, a process for the production of the air electrode bodies, and a process for the production of the solid oxide fuel cells.
2. Related Art Statement:
Since the solid oxide fuel cells (SOFC) operate at high temperatures of 1,000.degree. C. or more, electrode reaction is extremely active. Therefore, a noble catalyst metal catalyst such as precious platinum need not be used at all. Further, their polarization is small, and outputted voltage is relatively high. Thus, the energy-conversion efficiency is far greater as compared with other fuel cells. In addition, since the constituent parts of the solid oxide fuel cell are all solid, the solid oxide fuel cell is stable, and has a long-life.
At present, it is considered that zirconia is a most promising material for constituting a solid electrolyte film of the SOFC, and a lanthanum based perovskite-type composite oxide is promising as a material for the air electrode body (See Energy General Engineering 13-2, 1990).
However, it is reported that if a solid electrolyte film made of zirconia is formed on a surface of the lanthanum based perovskite-type composite oxide having a stoichiometrical composition, the zirconia reacts with perovskite composite oxide at around 1,200.degree. C. to produce La.sub.2 Zr.sub.2 O.sub.7 (See Proceedings of SOFC-NAGOYA 87-92, 1989). This fact means that when a laminated structure between an air electrode body and a solid electrolyte is to be formed, a high resistance layer made of La.sub.2 Zr.sub.2 O.sub.7 is produced between the air electrode body and the solid electrolyte on heating at not less than 1,200.degree. C., which largely reduce outputs from the cell.
As the structure of the SOFC unit cells, a self-supporting type structure is preferred, in which an air electrode body itself are used as a support and a solid electrolyte film and a fuel electrode film is formed thereon. As compared with a case in which an air electrode body film is formed on the surface of a porous ceramic support, the self-supporting type structure has the merit that the entire structure is simple, the producing process may be simplified, cost reduction is possible, and loss due to the gas-diffusing resistance is diminished to raise outputs from the unit cell. However, when the oxide electrolyte film is to be formed on the surface of the air electrode body, La.sub.2 Zr.sub.2 O.sub.7 is formed at the interface.